Backreaming tool

ABSTRACT

The invention provides a backreaming tool for expanding a pilot bore produced by a drill string, comprising a pneumatic perCussion hammer ( 1 ) in combination with a drill string, the drill string being rotatable and reversible through the pilot bore to effect backreaming by the pneumatic perCussion hammer. The pneumatic percussion hammer is longitudinally adjustable relative to the drill string. The arrangement is such that the compressed air supply to the working cylinder of the hammer is cut off when the hammer is longitudinally close to the end of the drill string, then restored as the hammer becomes more distant from the drill string.

[0001] This invention relates to a backreaming tool for expanding a pilot bore produced by a drill string (set of drill rods joined together in line).

[0002] Directional drilling, in particular horizontal drilling, is used for producing bores which can accommodate pipes, utility lines and the like without the need for continuous excavation. A drill string is first used to drill a pilot bore. This bore is then enlarged so as to accommodate the required pipe, utility line etc. Enlarging is conveniently carried out by backreaming, which involves attaching a backreaming tool to the front end of the drill string which projects from the pilot bore, then pulling the drill rod in reverse direction through the pilot bore while rotating the drill string. As the backreaming tool is pulled through the pilot bore on the end of the drill string, it enlarges and compacts the wall of the bore.

[0003] Various backreaming tools have been proposed, one example being shown in U.S. Pat. No. 5,921,331. While such tools may work satisfactorily in relatively soft ground, problems are encountered when the ground consists of continuous or intermittent rock, boulders etc. Such backreamers tend to jam in the ground or progress forward only very slowly, and therefore grind themselves away when confronted with rock and boulders.

[0004] WO 99/19596 describes a directional drilling tool which is effective for directional drilling in rock. It also describes a backreaming tool in which a drill string is used in combination with percussion hammers.

[0005] Percussion hammers (also known as “moles”) are known in the excavating field. They are commonly pneumatically operated, although other driving means are also known. It is also known for such percussion hammers to be drivable both forwardly and in reverse. An example of a percussion hammer with a reversing mechanism is shown in WO 91/19073.

[0006] The present invention provides a backreming tool for expanding a pilot bore produced by a drill string, comprising a pneumatic percussion hammer in combination with a drill string, the drill string being rotatable and reversible through the pilot bore to effect backreaming by the pneumatic percussion hammer;

[0007] wherein the pneumatic percussion hammer has a first end adjacent the forward end of the drill string, and a second end remote from the drill string, a compressed air inlet pipe which passes through the first end and into a working cylinder of the hammer, whereby the working cylinder is capable of reciprocating and exerting hammer blows on an internal end surface of the hammer at the first end;

[0008] wherein the compressed air inlet pipe is connected to the forward end of the drill string, for supply of compressed air through the drill string into the air inlet pipe, and the compressed air inlet pipe is connected to the pneumatic percussion hammer in longitudinally slidable manner and in rotatably fixed manner, whereby the pneumatic percussion hammer is longitudinally adjustable relative to the drill string but is rotatable in accordance with rotation of the drill string;

[0009] and wherein the position of the air inlet pipe relative to the working cylinder is such that the compressed air supply to the worldng cylinder is cut off or set in the neutral position when the hammer is longitudinally close to the end of the drill string, then restored as the hammer becomes more distant from the drill string, so as to prevent hammer blows falling on the forward end of the drill string.

[0010] The invention also provides a method of expanding a pilot bore which has been produced by a drill string, which comprises connecting a pneumatic percussion hammer to the forward end of the drill string, supplying compressed air through the drill string to the pneumatic percussion hammer and reversing the drill string through the bore while rotating the drill string, so as to effect backreaming of the pilot bore by the pneumatic percussion hammer;

[0011] wherein the supply of compressed air actuates the pneumatic percussion hammer so as to create hammer blows towards the drill string; and

[0012] wherein the compressed air supply to the pneumatic percussion hammer is cut off or set to the neutral position when the hammer is longitudinally close to the end of the drill string, then restored as the hammer becomes more distant from the drill string, so as to prevent hammer blows falling on the forward end of the drill string.

[0013] A shock damping device between the hammer and the drill string may also be employed to further ensure that the hammer blows do not strike the drill string.

[0014] Reference is now made to the accompanying drawings, in which;

[0015]FIG. 1 is a diagrammatic side view of a backreaming tool according to an embodiment of the invention, in which the compressed air supply to the pneumatic percussion hammer is cut off;

[0016]FIG. 2 is a view corresponding to FIG. 1, in which the compressed air supply has been restored so as to start the hammer blow cycle;

[0017]FIG. 3 is a view corresponding to FIG. 2, showing the position where the hammer blow occurs;

[0018]FIG. 4 is a side view of a backreaming tool according to an embodiment of the invention, showing external structural features;

[0019]FIG. 5 is a diagrammatic representation of heads of varying size which may be used in the backreaming tool;

[0020]FIGS. 6 and 7 are front and rear views of a head for use with the backreaming tool; and

[0021]FIGS. 8 and 9 represent further embodiments of head which may be used.

[0022] Referring first to FIGS. 1, 2 and 3, there is shown a pneumatic percussion hammer 1 which forms part of the backreaming tool according to the invention. The pneumatic percussion hammer 1 is itself of conventional design. It has a front end 2 and a rear end 3, and is connected at its rear end 3 to the front end of the drill string (not shown). In the embodiment shown, the pneumatic percussion hammer 1 is adapted to operate in “reverse” mode, i.e. the hammer blows are directed towards the rear end 3. However, the invention is not restricted to this arrangement, and it would be possible to have the front end of the pneumatic percussion hammer in connection with the forward end of the drill string, and the pneumatic percussion hammer would then run in “forward” mode.

[0023] The pneumatic percussion hammer has a hammer member 4 which is longitudinally slidable within a cylindrical chamber 5. The hammer member 4 has an inner working cylinder 6. Passages 7 communicate between the working cylinder 6 and the cylindrical chamber 5. A compressed air inlet pipe 8 passes through the rear end 3 of the pneumatic percussion hammer and into the working cylinder 6. The compressed air inlet pipe 8 is open at its forward end 9 so as to supply compressed air into the working cylinder 6.

[0024] The compressed air inlet pipe 8 is fixed at its rear end 10 to the forward end of the drill string, by a screw fitting or the like. Compressed air is supplied through the drill string and into the compressed air inlet pipe.

[0025] The compressed air inlet pipe 8 has a longitudinally splined or keged collar intermediate its ends, which constitutes a male member in cooperation with an internally splined or keged female member which constitutes part of the rear end structure of the pneumatic percussion hammer. This splined or keged connection enables the compressed air inlet pipe 8 to be longitudinally slidable in relation to the pneumatic percussion hammer 1. As the compressed air inlet pipe 8 is fixed to the drill string, the pneumatic percussion hammer 1 is itself longitudinally adjustable relative to the drill string. The splined or keged connection between the compressed air inlet pipe 8 and the pneumatic percussion hammer 1 also means that rotation of the drill string will cause corresponding rotation of the pneumatic percussion hammer 1.

[0026] The front end portion of the compressed air inlet pipe 8 has a control valve 11, which is a portion of increased diameter slidable within the bore of the working cylinder 6. Depending on the longitudinal position of the compressed air inlet pipe 8, the control valve 11 is capable of covering the ends of the passages 7 so as to prevent communication between the working cylinder 6 and the cylindrical chamber 5.

[0027] In the position shown in FIG. 1, the control valve 11 has been pushed to its forwardmost position, i.e. the pneumatic percussion hammer 1 is relatively close to the end of the drill string. In this position, the control valve 11 covers the passages 7, and the hammer 4 is against the forward internal end of the cylindrical chamber 5. Movement of the hammer 4 is stopped in this position. As the drill string continues its reverse passage through the pilot bore, the connection between the drill string and the rear end 10 of the compressed air inlet pipe 8 begins to pull the control valve 11 rearwards, because of the drag on the outer surfaces of the pneumatic percussion hammer against the sides of the bore. The position shown in FIG. 2 is then reached. The control valve 11 has travelled to the rear of the passages 7, so that these passages are no longer covered, and compressed air can pass from the working chamber 6 through the passages 7 and into the cylindrical chamber 5. The forward part of the hammer 4 has a side channel (not shown) permitting the compressed air to pass to the front part of the cylindrical chamber 5. The compressed air then acts on the front surface of the hammer 4, so as to drive it towards the rear, until the back surface of the hammer 4 impacts on the internal rear end of the cylindrical chamber 5, as shown in FIG. 3. At this point, the passages 7 have now passed to the rear of the control valve 11, and compressed air now passes in the reverse direction through these passages into the rear part of the working cylinder 6, and then to the outside of the pneumatic percussion hammer through exhaust channels 12.

[0028] The cycle then repeats itself, with compressed air entering the working cylinder 6 through the forward end of the compressed air inlet pipe 8. The resulting series of backward hammer blows causes the pneumatic percussion hammer 1 to begin to catch up with the drill string in its reverse passage through the pilot bore. When the pneumatic percussion hammer 1 again approaches the forward end of the drill string, the control valve 11 is again pushed in and the hammer cycle stops. In this way, hammer blows are prevented from being generated when the pneumatic percussion hammer is close to the drill string.

[0029] During the phase when the pneumatic percussion hammer is catching up with the reverse passage of the drill string through the pilot bore, the pneumatic percussion hammer is exerting hammer blows in the reverse direction. This enables the pneumatic percussion hammer to perform a backreaming action. A head 13 in the form of a ring shaped member is fitted on the rear end 3 of the pneumatic percussion hammer. The head 13 preferably has a flat rear face. This face may have hardened teeth 14 thereon, as shown in FIG. 4. The head 13 is a detachable member, and different size heads 13 may be provided as shown in FIG. 5 for different ground conditions and boring requirements.

[0030]FIG. 4 also shows a screw connection 15 between the rear end 10 of the compressed air inlet pipe and the forward end 16 of the drill string. Also shown in FIG. 4 is an outer casing 17 for the pneumatic percussion hammer, which enables it to be used when water is present in the pilot bore, without ingress of water preventing proper operation. Alternatively, the hammer can be constructed with one-way air exhaust passages to prevent the inlet of water.

[0031] FIGS. 6 to 9 show a variety of designs for the cutting head 13. Passages may be provided, facing forward and rearward, and optionally at an angle to the axis. These permit air or fluids such as water, bentonite or foam to pass through the head. Larger passages may also be provided in the periphery, as shown at 18, or closer to the centre, as shown at 19, for spoil removal. The head can also be of a star shape as shown in FIG. 8. A reduced or non-interrupted peripheral surface causes powdered spoil to be forced by the air and compression into fissures, voids etc. in the ground, while the uninterrupted surface smooths the bore.

[0032] The working face of the head 13 is preferably flat. This enables the greatest force to be applied to the ground, and in particular to rock and boulders. It ensures that the material is fully cut and powdered before being able to pass back over or through the head passages. This is in contrast to conventional reamers which are generally tapered and tend to jam in the ground or almost stall, and then grind themselves away when confronted with rock and boulders. They also tend to dislodge whole or parts of the rock which are passed through the blades or cutting wheels and lay in the bore, or they dislodge the bore completely causing collapse. The design of the present backreaming tool thus overcomes such problems. 

1. A backreaming tool for expanding a pilot bore produced by a drill string, comprising a pneumatic percussion hammer (1) in combination with a drill string, the drill string being rotatable and reversible through the pilot bore to effect backreaming by the pneumatic percussion hammer; wherein the pneumatic percussion hammer has a first end (3) adjacent the forward end of the drill string, and a second end (2) remote from the drill string, a compressed air inlet pipe (8) which passes through the first end and into a working cylinder (6) of the hammer, whereby the working cylinder is capable of reciprocating and exerting hammer blows on an internal end surface of the hammer at the first end; wherein the compressed air inlet pipe (8) is connected to the forward end of the drill string, for supply of compressed air through the drill string into the air inlet pipe, and the compressed air inlet pipe is connected to the pneumatic percussion hammer in longitudinally slidable manner and in rotatably fixed manner, whereby the pneumatic percussion hammer is longitudinally adjustable relative to the drill string but is rotatable in accordance with rotation of the drill string; and wherein the position of the air inlet pipe (8) relative to the working cylinder (6) is such that the compressed air supply to the working cylinder is cut off when the hammer is longitudinally close to the end of the drill string, then restored as the hammer becomes more distant from the drill string, so as to prevent hammer blows falling on the forward end of the drill string. A shock damping device between the hammer and the drill string may also be employed to further ensure that the hammer blows do not strike the drill string.
 2. A backreaming tool according to claim 1, also comprising shock damping means between the hammer and the drill string.
 3. A backreaming tool according to claim 1 or 2, wherein the pneumatic percussion hammer has a flat-faced head at its second end (2).
 4. A method of expanding a pilot bore which has been produced by a drill string, which comprises connecting a pneumatic percussion hammer (1) to the forward end of the drill string, supplying compressed air through the drill string to the pneumatic percussion hammer and reversing the drill string through the bore while rotating the drill string, so as to effect backreaming of the pilot bore by the pneumatic percussion hammer; wherein the supply of compressed air actuates the pneumatic percussion hammer so as to create hammer blows towards the drill string; and wherein the compressed air supply to the pneumatic percussion hammer is cut off when the hammer is longitudinally close to the end of the drill string, then restored as the hammer becomes more distant from the drill string, so as to prevent hammer blows falling on the forward end of the drill string. 